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Discussion Starter #1
My STI has been having a very odd lean spot/buck ever since I installed injectors and a walbro. I was hoping someone here could provide some alternative insight as what is going on, and what the culprit could be.

Car:
08 STI
DW 850cc's
Walbro Pump
Cobb AP, protuned
AEM CAI
P&L TBE(catless)
Perrin BOV

Symptons:

Ranging from 2500 to 3000 rpm(and usually stopping at exactly 3k) when cruising on the hwy usually in 5th and 6th and applying a light load, the engine will begin to buck/stumble. Its very odd and when looking at my data logs the one that stands out is the AFR. As soon as the bucking hesitation starts, the AFR's jump from 14 to 15 to 16 and sometimes 17. Once I hit 3k rpm afr's would go back to normal and the car would seem fine.

Another way I can tell is in start stop traffic, the car really doesnt like being shift below 3k rpm, and if I rev match to a lower gear, u can feel it stumble a bit if its below 3k.


Attempts to fix:

When the symptom first came about, Stephen at IAperformance and I spent countless hours etuning the car with no real result. After contacting other tuners, Stephen recommended I revert the current FPR system on my car to that of an 07 STI. We tried this and not much had happened. Finally, my housemate who is the local OS tuner decided to give it a whirl w/ AccessTuner Race. I figured this may net a better result since this time the tuner will actually be in the car and it could be easier to diagnose. We spent some time road tuning using the cobb stg 2 v104 maps as a base and voila it was gone.

Fast forward to now, roughly 3 months later, I'm driving home from getting the car serviced and the stumble/hesitation is back. The car was just getting an oil change and a tech inspection, I was there the whole time w/ the mechanic as well, so I know nothing other than an oil change and tech inspection occured.

When I got home I cleaned the MAF, changed the spark plugs(was going to do it that day anyway), reset the ecu, and ran an older map. Stumble was still there. So I reset the ecu again and ran the most current map, for a while it seemed as if the stumble had disappeared, but then it came back about an hour later.

The only thing I can think of that changed in the last 3 months is the weather. I live in Pensacola and the summer weather has finally arrived with temps in the upper 80s to mid 90s.

Like I said earlier, if anyone has any ideas as to what is causing this problem and has a possible solution please post up. While researching I read that it could also be a clogged PCV valve(I'm going to pick one up tomorrow and install it this weekend).
 

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Cruising with 16 and 17...man, try to avoid it until you short things out. You are slowly but surely killing the engine!

Get a log with:

Αtmospheric pressure
AFR
Ignition total timing
Learned ignition timing
Fine learning knock correction (FLKC)
Injector duty cycle
Fuel injector pulse width
Long term fuel trim
Intake air temp
Manifold relative pressure (corrected)
Mass airflow
Primary wastegate duty cycle
Throttle opening angle
Open/Closed loop fueling
Engine Speed
Vehicle speed
Coolant temp



Hoe does the car feel on WOT? (3rd/4th gear 2500~6500rpm) AFR?
 

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Discussion Starter #3
Well if I'm cruising AFR's are generally around stoich in closed loop. Its just when I apply that tiny bit of load to get from like 65 to 70 mph does it go crazy.

WOT Pulls are great, afr's are around 11.0 to 11.1, car pulls wonderfully, no complaints about that whatsoever. I'll post up a datalog when I get home this afternoon of the lean condition.
 

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i have the same exact issue seems like it only happens in S or i mode. very noticable hesitation 25-3k and my apr also jumps to around 16. its driving me crazy cause some days the car drives fine no stumble. vacuum leak? barometric pressure? i also heard that reverting to the older fuel system helps but it seems like it didnt work for u. in for some solution ideas..
 

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Discussion Starter #6
Alright my housemate and I played around some on the road today. We used the engine load compensation table posted by Airboy here(first post of the page): RomRaider • View topic - How to fix the 08 STi 2400-3200rpm stumbles

We used the same values for tables A and B since the car seems to want to switch between open loop and closed loop. This managed to eliminate 98% of the stumbling, theres still a bit, however tracking down every single load cell it does it at would take the better part of a day.
 

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Just about forgot this thread entirely - around the time of the last comment here, I was informed by a tuner from Bethel, Ct that this issue is likely resolved once and for all, for 2008+ STI's. Past corrections were not 100% resolved...but ~ 90%, likely the same for most folks w/ issue.

My observations prior to fix were that during WOT it was rarely felt, or a bother, but during normal/standard shifting it was noticable but minor one - more annoying that it wasn't gone. However, it was much less noticable if the air was very cool. Anyhow, received the new AP map and behold..done..warm or cold air temp no more stumble. I'd had the map for nearly 3 months now and glad it's no more.

Thought I would share this comment for those that still have the issue, or maybe are not aware of this issue with 2008+ STI.
 

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So what is the solution? Do I need to go get a tune in Ct to find out? Cheers JD
I'm no tuner, so I don't know the details of the correction. I only know of the previous annoying symptoms and corrected results. It's been superb since!
I still communicate with my tuner, as he is dilligent in ensuring all his tunes remain running as ought to be. :tup:
 

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You don't need a tune in CT. Read the thread above. It tells you exactly what to do. In fact, since it requires hours of highway datalogging most tuners will not address the issues since it varies slightly from car to car. You can transfer a lot of the engine load table from a generic tune and have it fix most of the stumbles, but to really get it correct for your car you need to log.
 

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Check out my replies in the AP thread above. I tried to make some step-by-step instructions to get it to work. It did take a little trial and error, but the results are very nice.
 

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Have you guys gotten this issue fixed? I found if i add 3' of hose between my fuel lines and fuel rails, i can 0 out the compensation tables and it completely gets rid of the 3k stumble in 08+ STi s. It's this weird thing with injector pulse and such.
 

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Yes, it is definitely a resonance issue. Did you just throw a 3' coil of fuel line under your hood? How did you plumb it, those fuel line fittings (from lines to rails) are not something I can source easily.
 

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Have you guys gotten this issue fixed? I found if i add 3' of hose between my fuel lines and fuel rails, i can 0 out the compensation tables and it completely gets rid of the 3k stumble in 08+ STi s. It's this weird thing with injector pulse and such.
Very interesting! I was hoping there'd be more discussion about this. How did you decide on 3 feet? Can you post a pic of what this looks like?
 

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Answering my own question: No pic, but more details and discussion with a high percentage of success: Page ~18 of the RomRaider thread.
Quoting the answer to my question because I might be retarded: I can't find any info on what quick connectors I'll need (on the engine side, not the FPR side) in order to install a longer length of fuel line.

Anyone?
 

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My additonal fuel damper arrived c/o IA Performance. Installed it in-line with the existing damper i previously got from them as well. Took out the 1m fuel hose and replaced it with the extra damper. Happy to report that the stumble is still absent. So for those who dont want the extra clutter that a coiled hose could impose on the engine bay, this is an alternative.




Sent from my iPhone using Tapatalk
 

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http://www.iwsti.com/forums/2-5-liter-litre-factory-motor/248575-aem-cai-ltft-iat-mystery-solved.html#post3690845

Works for me. Zero stumble with no changes to the fuel system. Injectors are still part of the equation, but in my case it is an electrical/electronic connection instead of fuel itself.

EDIT

After reading further in the Romraider thread about fuel line coils and dampers, it is clear I was uninformed when I posted the above.

While I may have made the difference to my car that my link points to, it is now clear there are mechanical issues at work here, too. Therefore, despite my own car's (apparent) freedom from the infamous stumble issue, I'm going to add a one meter coil of fuel injector hose to address the mechanical problem for peace of mind. Given the design of our fuel system there is good reason to believe such a "fix" as the 100% effective coil of hose will make a positive change to my car as well. And while I may not presently experience any stumble, it is only logical that a positive substantive change will be made and will be revealed during future analysis of performance.
 
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